1. Field of the Invention
The present invention relates to a color conversion apparatus in use with a digital image processing system.
2. Discussion of the Background
There have been known various types of digital color image processing systems, such as a digital color copying machine, color facsimile, and color display. The image processing systems have been widely used, while involving problems to urgently be solved. Some of the problems are found in the color conversion, ".gamma." conversion, and operation in a mono color mode.
The color conversion problem will first be given. Japanese Patent Application Laying Open Publication No. 58-142669 disclosed a typical color conversion apparatus in a digital color image processing system. In the color conversion apparatus, a color image on an original is read in terms of separated color image signals. The separated color signals representing color image data are stored. One of the correspondences of the image signals as read and the colors of ink are selected. By using the selected ink and image signal, an image based on the selected image signal is printed on a recording medium.
The color conversion apparatus is thus based on the selection of the correspondence between the read image data and the inks used for image printing. Varieties of the colors that can be converted are limited to R (red), G (green) and B (blue) or Y (yellow), M (magenta) and C (cyan). Therefore, other colors than a color to be converted (referred to as an original color) are often contained in a color it is converted (color after converted). Prediction of the color to be converted is uncertain.
The digital image processing system based on the color conversion apparatus is currently being marketed. In the system, the color to be converted is lack of gradation data in connection with the original color. Accordingly, the original color and the colors located in the vicinity of the original color (in a color space), when converted, become all the same color.
The ".gamma." conversion problem may be found in Japanese Patent Application Laying Open Publication No. 59-161970. In the image processing system of this gazette, three ".gamma." correction circuits are provided for the separated color signals R, G and B and are provided and operated referenced to ROMs. Since the 2 high-order bits in the ROMs may be used for controlling the ".gamma." correction, the ".gamma." conversion is simultaneously carried out. To effect the ".gamma." conversion, a multiplier may be used. In this case, however, a value correction cannot be directly applied to the separated color signals R, G and B. Further, additional calculation works are needed and a complicated circuit arrangement is also required.
The mono color problem may be found in the image processing system disclosed in Japanese Patent Application Laying Open Publication No. 58-142670. In a mono color mode, the system reads the separated color image data of R, G and B from a memory one time or a plurality of times, and uses the read image data for printing. Accordingly, the number of colors to be printed in a mono color mode is limited to the primary colors (R, G, B) or the secondary colors (Y, M and C) and tertially color (Bk). However, recent market demands for the mono color print are to obtain an image of a desired single color.